| 摘要 |
Tuning control apparatus is provided for use with a frequency synthesizer tuner of the type having a phase-locked loop including a reference oscillator, a variable frequency oscillator to produce a local oscillating signal, a programmable frequency divider coupled to the variable frequency oscillator for dividing the frequency of the local oscillating signal by a variable dividing ratio to produce a frequency-divided oscillating signal, a phase comparator for comparing the frequency-divided oscillating signal to the output of the reference oscillator to produce an error signal, and a feedback circuit for feeding back the error signal from the phase comparator to the variable frequency oscillator to adjust the frequency of the local oscillating signal and thereby establish a tuning condition of the tuner. The tuning control apparatus includes a counter for counting control pulses supplied thereto, the count of this counter determining the dividing ratio of the programmable frequency divider. A control pulse generator supplies control pulses to the counter. An adjusting circuit is operable to adjust the rate at which the control pulses are supplied to the counter for adjusting the speed at which the count of the counter and the dividing ratio of the programmable frequency divider changes. In one embodiment of this invention, a switching circuit is provided to be selectively operable to supply only a predetermined number of the control pulses to the counter so as to change the count of the latter, and thus the dividing ratio of the programmable frequency divider, by a predetermined step. Thus, the tuning condition of the frequency synthesizer tuner can be changed at a variable rate or, alternatively, in a stepwise manner.
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